Electrical insulator and conducting tar therefor

ABSTRACT

A conducting tar coating is placed on the surfaces of the conductive hardware of an insulator, and on the surfaces of an insulator shell connected to the hardware by a cement. The insulator surface has a semiconducting coating. The conducting tar is formed by admixing carbon black in an asphalt base after dissolving the asphalt with a suitable solvent.

United States Patent [191 Rosenblatt ELECTRICAL INSULATOR AND CONDUCTING TAR THEREFOR [75] Inventor: Gordon Rosenblatt, Burlington,

Ontario, Canada [73] Assignee: Canadian Porcelain Company Limited, Ontario, Canada [22] Filed: Dec. 14, 1972 [21] Appl. No.: 315,119

I [5 2] US. Cl 174/140 C, 106/277, 106/278, 174/182, 174/211 [51] Int. Cl. H011) 17/42, HOlb 17/50 [58] Field of Search; 174/140 R, 140 C, 141R, 174/141C,182,186,188, 189,196, 209, 211

[56] References Cited UNITED STATES PATENTS 10/1934 Austin, 174/182 Sept. 17, 1974 1,994,293 3/1935 Taylor 174/182 2,146,344 2/1939 Meisse 2,173,292 9/1939 Austin 174/196 X FOREIGN PATENTS OR APPLICATlONS 586,065 3/1947 Great Britain 174/140 C Primary Examiner-Laramie E. Askin Attorney, Agent, or Firm-Ostrolenk, Faber, Gerb & Soffen [5 7] ABSTRACT A conducting tar coating is placed on the surfaces of the conductive hardware of an insulator, and on the surfaces of an insulator shell connected to the hardware by a cement. The insulator surface has a semiconducting coating. The conducting tar is formed by admixing carbon black in an asphalt base after dissolving the asphalt with a suitable solvent.

11 Claims, 2 Drawing Figures ELECTRICAL INSULATOR' AND CONDUCTING TAR THEREFOR BACKGROUND OF THE INVENTION ally consist of two conductive hardware members fastened by a cement to opposite surfaces of a suitably contoured-insulator shell. The hardware members, typically an upper metalcap and a lower metalpin, are secured to the opposite surfacesof the insulator shell by a layer of cement. Commonly, however, the insulator surfaces and conductor surfaces are first painted with a tar or thin, strongly adherent layers of asphalt, which provides a cushion actionto absorb forces due to thermal expansion and mechanical shock which might appear between the cement and the insulator shell or conductive hardware.

In order to improve the performance of the insulator, a semiconducting glaze can be applied to the insulator shell surface. This semiconducting coating then carries a small current which tends to heat the i'nsulatorsurface so that it can perform better in various adverse environments. When a semiconducting glaze is used, it isnecessary to make an electrical connection between the insulator shell surface and the conductive hardware. This connection has been made in a variety of ways, typically by metal layers sprayed over the nonconductive cement and tar layers to form a direct conductive path from the insulator shell to the hardware.

BRIEF SUMMARY OF THE INVENTION In accordance with the present invention, the tar used to coat the insulator shell surface and hardware surface is loaded with a conductive material such as carbon black to enable a direct electrical path to be formed from the insulator hardware to the semiconducting surface of the insulator shell. The cement which joins the hardware to the insulator shell should also be conductive. Thus, the present invention avoids the prior complex electrical connectors-needed to by pass the conventionally insulating tar paint.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a suspension type insulator which uses the conductive tar of the invention.

FIG. 2 is a diagram showing surface resistivity as a function of carbon loading.

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIG. 1, there is shown a suspension insulator which has conductive hardware including an upper metal cap and a bottom metal pin 11, where cap 10 and pin 11 are formed conventionally so that their insulators can be secured in a string. An insulator shell 12,;conventionally of porcelain, is cemented between members 10 and 11. This is the conventional form of a suspension-type insulator. It will be understood by those skilled in the art that the invention'disclosed hereinafter is not limited to'use with suspension insulators, but can be used in any type of insulator structure.

The interior surface of metal cap 10 is coated with a thin bituminous coating 20. In a like manner, the outer surface of metal pin 11 is coated with a thin bituminous coating 21..The outer cylindrical surface 22, and inner cylindrical surface 23 of insulation shell. 12, are formed by porcelain sand layers, and these layers are in turn coated with thin bituminous coatings 24 and 25 respectively. The bituminous coatings 20, 21, 24 and 25 are strongly adherent to their respective surfaces.

The cap 10 is then secured to shell 12 by cement layer 30 while pin 11 is secured to shell 12 by cement layer 31.

The above structure is generally old where the coatings 20, 21, 24 and 25 and cement layers 30 and 31 have insulation characteristics. As previously stated, it is known that shell 12 can have a semiconducting glaze 40 over its outer surface. In the past, where such a glaze isused, metal layers sprayed over the nonconductive cement layers 30 and 31 and the bituminous coatings 20, 21, 24 and 25 have been used to connect the glaze surface 40 to the cap 10 and the pin 11.

In accordance with the present invention, it has been found that the coatings 20, 21, 24 and 24 can be made conductive without adversely affecting their shock resistant qualities. Thus, by also making the cement layers 30 and 31 conductive, as by loading carbon black or graphite fibres into the cement, the semiconducting glaze 40 is connected to both the cap 10 and pin 11 without the need for other connection means.

Generally, it has been found that the addition of conductive materials, such as carbon black or graphite powder in the tar, can increase the tar conductivity to acceptable levels for use as a conductive path from semiconductor glazeto metal hardware. The use of carbon black has been found superior to the use of graphite powder, and, in particular, carbon black used to produce conductive rubber or plastic has been found satisfactory in that it does not adversely interfere with the desired mechanical quality of the tar when used in certain proportions.

Two brands of carbon black which were tested are:

Conductex SC, made by Columbian Carbon Ltd., and Vulcan XC-72, made by Cabot Carbon Ltd. The Vulcan XC-72 is preferred in this application since it produces a more conductive film at lower carbon load- One typical formulation of a tar which was successfully used to form layers 20, 21, 24 and 25 in FIG. 1 was:

Carbon (Conductex SC) 37.5% (by weight) Tar (Venezuelan Crude Asphalt) 62.5% (by weight) the lower limit is determined by the minimum acceptweight) produceda more conductive coating than 37.5

percent Conductex SC. Typical readings of the two carbon varieties are listed below. Resistances were measured on dry coated slides by an ohmmeter with probes spaced one inch apart on a one-inch wide strip:

37.5% Conductex (in tar) 32.5% Vulcan XC-72 (in tar) 80,000 ohms 20,000 ohms The coatings were found to be adherent to ceramic substrates, including glazes or glass and also possess other necessary properties such as flexibility and resistance to cracking. Note that this type of coating could find uses in other areas than insulator manufacture, for example, as a general resistance coating. Since the base of the coating is asphalt, a high coefficient of thermal expansion is provided which is an advantage in an insulator application.

In order to prepare the tar, the tar component is dissolved in a suitable solvent, e.g., a mineral solvent such as naphtha. Other solvents could be used, such as carbon tetrachloride or benzol, taking appropriate precautions in the handling of these materials.

The solvent is preferably one having a low evaporation rate. Thus, the solvent will usually evaporate in three to five hours. in practice, abouttwelve hours of air drying is used to complete the evaporation process.

In one formulation, to form a tar with 37.5 percent by weight of Conductex SC, the following products were'used:

Carbon (Conductex SC) Tar (Venezuelan Crude Asphalt) Naphtha (Technical Grade) 10.6% (by weight) 17.6% (by weight) 71.8% (by weight) Another formulation using Vulcan XC-72 in which the carbon formed 32.5 percent by weight of carbon and tar'was:

Carbon (Vulcan XC-72) Tar (Venezuelan Crude Asphalt) Naphtha 9.0% (by weight) 18.7% (by weight) 72.3% (by weight) ing its .surface resistivityin ohms per square. Note that the surface resistivity reduced from about 2 X ohms/square to about 3.5 X 10 ohms/square as the carbon loading increased from about 27.5 percent to about 35 percent by weight of carbon and t'a'rf The paint is prepared by, dissolving the tar. component in part of the naphtha after which it is mixedwith a naphtha-carbon mixture, which is made by soaking the total batch of carbon in naphtha overnight. This mixture is milled in a ball mill for one hour and the remainder of the naphtha is added and milled for a further three to six hours. Samples of the paint are taken by dipping glass slides and the end point for the milling can be then determined by a conductivity measurement on the coating; A ball mill or other high shear mixer must be used in the preparation of the paint inorder to produce a homogeneous mixture. Y

A layerof the paint is then applied, as by'painting, to

the surfaces of the insulator shell 12 and conductive hardware members 10 and 11 and is air dried. They could also be dried with warm air heaters. The layer could also be applied by dipping, or by any other desired process. The layer is applied to a thickness of about 1.0 mi] and in a range of from 0.5 to 2.0 mils.

' Although this invention has been described with respect to its preferred embodiments, it should be understood that many variations and modifications will now be obvious to those skilled in the art, and it is preferred, therefore, that the scope of the invention be limited, not by the specific disclosure herein, but only by the appended claims.

The embodiments of the invention in which an enclusive privilege or property is claimed are defined as follows: I

1. An electrical insulator comprising, in combination: an insulation shell member having a semiconducting surface, a conductive hardware member, a cement layer cementing a first surface region of said insulation shell member to a first surface region of said hardware member and a conductive tar coating adhering to said first surface region of said insulation shell member and disposed between said first surface region of said insulation shell member and said cement layer; said conductive tar coating being in electrical surface contact with said semiconducting coating, said semiconducting surface being electrically connected to said conductive hardware member.

2. The electrical insulator of claim 1 wherein said cement layer is conductive.

3. The electrical insulator of claim 2 wherein a second tar coating is formed on said first surface region of said conductive hardware member and is disposed between said first surface region of said hardware member and saidcement layer.

4. Theelectrical insulator of claim 2 wherein said tar coating consists of an asphalt base having carbon black admixed therein.

5. The electrical insulator of claim 1 wherein said tar coating consists of an asphalt base having carbon black admixed therein.

6. The electrical insulator of plaim 5 wherein said tar coating contains carbon black from 20 percent to 40 percent by weight of said tar plus carbon.

7. The electrical insulator of claim 6 wherein said tar coating contains about 32 percent to 38 percent by weight of carbon black.

'8. An electrical insulator comprising first and second metal members,an insulation member having a semiconducting glaze coating, a conductive tar coating on respective given surface regions-of said first and second metal members and on first and second spaced regions of' said' insulation member and in contact with .said semiconducting glaze coating, and first and second conductive cement layers connecting said given surface regions of said first and second metal members to said tar coating consists of a painted asphalt layer having carbon black admixed therein.

11. The insulator of claim 10 wherein said carbon black forms from 20 percent to 40 percent by weight of said conductive tar plus carbon. 

2. The electrical insulator of claim 1 wherein said cement layer is conductive.
 3. The electrical insulator of claim 2 wherein a second tar coating is formed on said first surface region of said conductive hardware member and is disposed between said first surface region of said hardware member and said cement layer.
 4. The electrical insulator of claim 2 wherein said tar coating consists of an asphalt base having carbon black admixed therein.
 5. The electrical insulator of claim 1 wherein said tar coating consists of an asphalt base having carbon black admixed therein.
 6. The electrical insulator of claim 5 wherein said tar coating contains carbon black from 20 percent to 40 percent by weight of said tar plus carbon.
 7. The electrical insulator of claim 6 wherein said tar coating contains about 32 percent to 38 percent by weight of carbon black.
 8. An electrical insulator comprising first and second metal members, an insulation member having a semiconducting glaze coating, a conductive tar coating on respective given surface regions of said first and second metal members and on first and second spaced regions of said insulation member and in contact with said semiconducting glaze coating, and first and second conductive cement layers connecting said given surface regions of said first and second metal members to said first and second spaced regions respectively of said insulation member.
 9. The insulator of claim 8 wherein said insulator is a suspension insulator and wherein said first metal member is a metal cap and wherein said second metal member is a metal pin.
 10. The insulator of claim 9 wherein said conductive tar coating consists of a painted asphalt layer having carbon black admixed therein.
 11. The insulator of claim 10 wherein said carbon black forms from 20 percent to 40 percent by weight of said conductive tar plus carbon. 